History
The text of O Come, O Come, Emmanuel is based on the biblical prophesy from Isaiah 7:14:

Therefore the Lord himself will give you a sign: The virgin will be with child and will give birth to a son, and will call him Immanuel.

The author of the carol is unknown, though the music seems to have been included in a 15th Century French processional for Franciscan nuns, though it too may have origins in 8th Century Gregorian Chant. Our present day English translation from Latin, was written by John Mason Neale in the mid-19th century.